Corrugated micropermeable membrane

ABSTRACT

Corrugated porous membranes made of polyvinylidene fluoride may be satisfactorily molded using a silane modified room temperature vulcanizable silicone rubber mold.

United States Patent [50] Field of Search [56] References Cited UNITEDSTATES PATENTS 2,997,448 8/1961 Hochberg... 260/25 M 3,177,637 4/1965Davis 210/493 Primary Examiner-Morris Sussman Attorneys-Brown andMikulka and Sheldon W. Rothstein ABSTRACT: Corrugated porous membranesmade of polyvinylidene fluoride may be satisfactorily molded using asilane modified room temperature vulcanizable silicone rubber mold.

b -ominopropyltriethoxysilone rigid support PAIENTEDIIUV 1s IHII 3,620,895

sum 1 or 3 IMPART TO A DEFORMABLE BASE MATERIAL THE CONFIGURATIONDESIRED 'IN ULTIMATE MEMBRANE.

COAT ONE SIDE OF THE DEFORMED BASE MATERIAL WITHU-AMINOPROPYLTRIETHOXYSILANE.

COAT A RIGID SUPPORT MEMBER WITH A RTV SILICONE RUBBER.

T CONTACT THE SILANE-COATED SIDE OF THE BASE MATERIAL WITH THE RTVSILICONE RUBBER- COATED MEMBER AND ALLOW THE SILICONE RUBBER TOVULCANIZE.

CONTACT THE RESULTANT COMPOSITE STRUCTURE WITH A SOLVENT FOR THE BASEMATERIAL WHICH IS A NONSOLVENT FOR THE OTHER COMPONENTS OF THESTRUCTURE, THEREBY REMOVING SAID BASE MATERIAL FROM SAID COMPOSITESTRUCTURE AND RESULTING IN A SILANE- MODIFIED SILICONE RUBBER MOLDMEMBER.

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INVENTORS JAMES Li BAILEY (In BY FRANKLIN A. RODGERS lfi /fiw rl, 0 :91fizz/( 41MB [ix/Mam WWI/M121 AT TO RN E YS PATENTEDnnv 1s I97l 3.620,895

sum 2 [IF 3 INVENTORS JAMES L. BAILEY and BY FRANKLIN A. RODGERS W wm h?ATTORNEYS It -ominopropyltriethoxysilune PAIENTEfluov 16 I9?! SHEET 3I]? 3 (00! x as) uogumguad 6 8 LI. n

m g o m 0 ID 2 I INVENTORS JAMES L. BAILEY and W m ATTORNEYS 1CORRUGATED MICROPERMEABLE MEMBRANE This invention is directed to castingfilms of polyvinylidene fluoride and, particularly, microporous filmswhich may be utilized in water purification.

In a practical separation process directed toward the extraction ofsubstantially pure water from an impure water solution, the energyrequired is related to the potentials causing transport of the extractedconstituent from the solution.

in order to effect separation of impurities from a volume of nonpotablewater it is necessary to physically separate the volume of water intoaliquots of different concentrations of impurities. One practicalapproach to the problem is to use a membrane which is relatively morepermeable either to pure water or to salts and other impurities whichare to be removed from the original water charge. If, for example, seawater and pure water, both'at the same pressure, are separated by amicropermeable membrane, the concentrations of the two liquids tend toequalize by passage through the membrane of impurity or water or both.If the membrane used is more permeable to pure water than impure water,the pure water will dilute the impure water. However, if a pure watermigration stimulus is applied on the impure water side of the membrane,pure water will pass from the impure water solution to the pure waterside at an appreciable rate. The energy required for this separationprocess may be supplied in the form of a hydrostatic head differential,wherein impure solution is delivered to the high pressure side of amembrane and is termed a reverse osmosis process; or in the form of avapor pressure differential accomplished by a heating element whichheats the impure solution and thereby raises the vapor pressureof thesolvent of the impure solution substantially, termed a membranedistillation process; etc. The primary difference between the reverseosmosis process and the membrane distillation process is the fact thatin the former, large hydrostatic pressure differentials are requiredwhich necessitates the use of a thick membrane of great strength, whilein the latter process, the hydrostatic heads on both sides of themembrane are substantially equal thereby allowing for the usage of athinner, more fragile membrane which provides less impediment to solventtransport.

Microporous films anticipated by the present invention are particularly,but not exclusively, adapted for use in the distillation apparatusdisclosed and claimed in copending application of Franklin A. Rodgers,Ser. No. 524,366, filed Dec. 27, 1967. The distillation apparatusgenerally comprises means for transferring heat to a first body ofliquid comprising a desired solvent (such as brackish water) to effecttransfer of solvent as a vapor across a barrier to a second body of thesame solvent from which heat is removed. The barrier is designed toseparate the two bodies of liquid so that there is no liquid flow, orleakage, from one to the other, while allowing the vapor of the solventto pass by diffusion from the evaporating liquid body, to which heat istransferred, to the condensing liquid body, from which heat iswithdrawn. The operational efficiency of the apparatus may be greatlyaffected by the nature and quality of the barrier layer which is used.Preferably, the film barrier will comprise a thin sheet of microporousmaterial having a multiplicity of microscopic through pores, orpassages, of substantially uniform size which occupy the major portionof the total volume of the film. The pores should be of a maximum sizewhich permit the passage of only the solvent vapor, and any gasdissolved therein substantially at its vapor pressure, without allowingthe passage of the liquid. The major proportion of the pores should beof maximum size to provide maximum efficiency with as few smaller sizedpores as possible. The smaller sized pores are undesirable in that thevapor will not readily pass therethrough and thus, the overall vaportransmission per unit area of barrier material is proportionatelydecreased by their presence, thereby decreasing efficiency. Pores of alarger size are intolerable since they may allow the passage of liquidand thereby prevent the apparatus from efficiently performing itsseparation function. The material used in the formation of the filmshould be nonwettable by the particular liquid for which the apparatusis designed and/or employed, and have a thermal conductivity as small aspossible, since heat-transfer between the bodies of liquid on oppositesides of the film should be restricted as much as possible to thatresultant from vapor transfer, rather than by direct conduction of thefilm. For the purification and desalination of brackish water, it hasbeen found that commercially available polymeric vinylidene fluoride maybe formed into effective barrier elements for use in distillationapparatus as described above. A method for the manufacture of suitablefilmsfor use in the herein described separation environment is disclosedand claimed in the U.S. application, Ser. No. 790,192 filed Jan. 3, 1969which is a continuation-in-part of U.S. application, Ser. No. 557,993,of James L. Bailey and Robert F. Mc- Cune, filed June 16, 1966, nowabandoned.

One of the basic processes used in the preparation of microporous filmsinvolves the admixture of a solvent solution of the film-formingmaterial with a liquid, which is a nonsolvent for the material and ismiscible with the solvent, and forming the mixture into a film. Thisprocess is denoted the solvent-nonsolvent process for formingmicroporous films. Many variations on the basic process are also knownsuch as those shown in U.S. Pat. Nos. 1,421,341; 3,100,721; and3,208,875. The solvent-nonsolvent" process is apparently based on thetheory that a polymeric material dissolved in a solvent and cast into afilm coalesces to form a film by the entanglement of polymer chains. Dueto the solvent action of the solvent still remaining in the film at thetime of coalescence, the polymer chains still have some freedom ofmovement so that the number of entanglements may be increased to renderthe film continuous and nonporous. By contacting the filmformingmaterial with a nonsolvent material which is miscible with the solventpresent in said material either prior to or subsequent to the formationof the material into a wet film, the solvent action of the solvent issubstantially decreased so as to limit chain entanglements and to thusprovide formation of a porous polymeric matrix.

According to the above-mentioned Bailey and McCune application, amicroporous film of polymeric vinylidene fluoride is provided having apore volume of at least about 50 percent and preferably at least 60percent wherein a majority and preferably at least about 75 percent ofthe pores have a pore diameter of from about 0.5 to about 2.0 micronsand less than about 5 percent of the pores have a pore diameter greaterthan about 2.0 microns. This microporous film may be prepared by forminga solution of polymeric vinylidene fluoride in a solvent therefor,subjecting said solution to a predetermined maximum temperature toeffect formation of a film having a given uniform pore sizedistribution, forming the solution into such a film, immersing the filmin a bath of a liquid which is a nonsolvent for the polymeric vinylidenefluoride and is miscible with the solvent, removing the film from thebath, and drying it. Preferably, the resultant microporous film is thenbaked, for a time sufiicient to anneal it, at a temperature insufficientto fuse the polymeric vinylidene fluoride. According to the disclosurein the aforementioned Bailey and McCune application, the ultimate porevolume and pore size distribution may be controlled by the judiciousselection of an appropriate maximum temperature to which thepolyvinylidene fluoride solution is subjected prior to casting. It willbe appreciated, therefore, that by adjusting the temperature within therange of 28 C. to 62 C. membranes having pore diameters with theabove-denoted desired 0.5 to 2.0 micron range will be produced.

When films of the aforementioned type are utilized in a separationapparatus such as that disclosed in copending above-denoted U.S.application of Franklin A. Rodgers, any scaling occurring at themembrane surface upsets the flow pattern of circulating liquid andgreatly decreases system efficiency, and reduces the average useful lifeof the separation membranes substantially. it has been found quiteunexpectedly, that adverse circulation problems due to scaling may besubstantially obviated by utilizing a membrane possessing multipleconvolutions on at least one side thereof, hereinafter referred to as acorrugated membrane.

it is, accordingly, a primary purpose of the instant invention toprovide a micropenneable membrane particularly adapted to be used in adistillation process for water purification.

It is another object of the present invention to produce a microporouscorrugated membrane.

It is a further object of the present invention to provide a techniquefor the production of uniform membranes described in the paragraph nextabove.

It is another object of the present invention to provide a novelpolyvinylidene fluoride membrane molding technique which produces acorrugated membrane.

It is a further object of the present invention to produce mold memberswhich may be reused for the formation of polyvinylidene fluoridemembranes wherein said mold members possess release characteristicswhich prevent premature peeling of the membrane from the mold member.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the several steps and the relationand order of one or more of such steps with respect to each of theothers, and the product possessing the features, properties and therelation of elements which are exemplified in the following detaileddisclosure, and the scope of the application of which will be indicatedin the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings.

FIG. 1 is a flow diagram of the process of the present invention;

FIG. 2 is a reproduction of a photomicrograph of a cross section of apolyvinylidene fluoride microporous membrane produced according to theprocess of the present invention;

FIG. 3 is a reproduction of a photomicrograph of a cross section of aprematurely released polyvinylidene fluoride microporous membraneproduced according to the process of the present invention with theexception that the silane application was omitted;

FIG. 4 is a graphic illustration of the pore size distribution of thecorrugated membrane of the present invention; and

FIG. 5 is a cross-sectional view of a mold member prepared according tothe present invention.

As has been stated above, in order to achieve satisfactory flowproperties within cell configurations in which the membrane of thepresent invention is particularly designed for use, the membrane shouldpossess flow-channeling convolutions. The utilization of the corrugatedmembranes of the instant invention in a distillation environment will bedisclosed and claimed in the application of Franklin A. Rodgers, Ser.No. 838,769, filed July 3, 1969.

Any attempt to mechanically corrugate a microporous membrane, such as,for example, that disclosed and claimed in the above-denoted copendingapplication of James L. Bailey and Robert F. McCune, causes anintolerable decrease in membrane efficiency since many of the pores arephysically collapsed. Accordingly, a system for producing a corrugatedmicroporous membrane was devised wherein said membrane is produced in acorrugated configuration in situ.

It has been found that excellent separation results may be achieved byemploying the distillation apparatus described in the above-denotedRodgers U.S. pat. application when the micropermeable membrane has apore volume of at least 50 percent andJpreferably greater than 60percent and the pore diameter distribution range falls substantiallybetween 0.5 to 1.5 microns with preferably more than 75 percent of thepores falling within this range and a majority of the pores fallingwithin the 0.5 to 1.0 micron range. Utilization of a membrane containinga pore size distribution within this range optimizes the resultsachieved in the above-denoted distillation environment under optimumoperating parameters but should not be considered a contraindication ofthe use of membranes with a pore diameter distribution range outside theabove-disclosed range since certain separation operations require porediameters of less than 0.5 micron. It will be appreciated that while apredominance of pores in the 0.5 to 1.0 micron diameter range isconsidered to provide the best mode of operation, adequate results maybe obtained when the membranes possess a pore size distribution whereinover 75 percent of the pores are in the 0.5 to 2.0 micron range withless than 5 percent of the pores having a diameter greater than themicrons. It will be accordingly evident that optimal results with agiven set of operating parameters may be achieved by selecting amicropermeable membrane comprising a narrow distribution of porediameters within the above-denoted 0.5 to 2.0 micron range.

The process of the present invention requires that any deformable basematerial which is capable of permanently retaining any deformationphysically imparted thereto be subjected to suitable deforming means,such as, for example, a thermomechanical means in the form of heatedrotating dies, heated platens, etc., which will impart to said materiala given number per unit length of convolutions of a predeterminedamplitude substantially identical to those desired in the ultimatelyproduced membrane. To provide satisfactory results in the above-denoteddistillation apparatus, ideally, the membrane should possess at leasteight convolutions per inch and preferably about 72 convolutions perinch. Since some shrinkage occurs, the amplitude of the convolutionsimparted to the base material should be slightly greater than thatdesired in the ultimate membrane. About 3 to 5 mils amplitude for theconvolutions in the base material provides a satisfactory amplitude tothe convolutions of the ultimate membrane. The amplitude of theconvolutions of the ultimate membrane should be about one-half thethickness of the membrane. Material such as the polycarbonates, e.g.,Lcxan; polyphenylene sulfone; polystyrene, etc. have been found ideal toprovide this function. Lexan polycarbonate is the preferred material.The corrugated base material is coated with a solution of a silanecoupling agent, which insures an adequate bond, specifically'y-aminopropyltriethoxysilane, in about a 2 percent solution. Thismaterial may be obtained from the Union Carbide Corporation under thedesignation Al I00 and may be used in concentrations of as little as0.25 percent. While any silane solvent may be used as long as it doesnot deleteriously effect the base material, the preferred solvent isisopropanol and application to the base material may be carried out byflow coating, swabbing, etc., to provide a thin, almost monomolecularlayer of the silane on the base material. The silane is allowed to dryon the base material and provides a coating thereon which identicallyfollows the contour of the convolutions thereof. Subsequently, a coatingof a room temperature vulcanizablc silicone rubber, such as Silastic RTV732,sold by Dow Chemical Company; types 102 or l08,sold by GeneralElectric Company; etc., is spread on a rigid member such as, forexample, a piece of glass, or the like, in a thickness of aboutone-thirty second inch, which is, by no means, critical, but must, ofcourse, be thicker than the amplitude of the convolutions imparted tothe base material, and the silane coated surface of the base material iscontacted therewith. The silicone rubber is used in the form of a onehundred percent solids material in the fluid state and readilycrosslinks at room temperature when it contacts ambient moisture. It isallowed to cure whereupon a composite structure results. The compositestructure is contacted with a solvent for the base material which, inthe case of Lcxan might be, for example, dichloromethane. The primeparameter in selecting the solvent for the base material is that it be anonsolvent for the remaining elements of the mold member. Other suchsolvents include ethylene dichloride, dimethyl acetamide, acetone, etc.Upon removal of the base material, the remaining mold member may beutilized according to, for example, the process of the above-mentionedBailey and Mc- Cune application, to produce a microporous corrugatedpolyvinylidene fluoride membrane. Briefly, this involves contacting thesilane surface of the mold member with a solution of polyvinylidenefluoride in a suitable solvent, such as dimethyl acetamide. Thepolyvinylidene fluoride coated mold member is then immersed in a bathwhich is a nonsolvent for the polyvinylidene fluoride but which ismiscible with the already present polyvinylidene fluoride solvent, forexample, the above-mentioned dimethyl acetamide, to produce a gelledmembrane Such nonsolvent materials include methanol, ethanol, propane]and other higher boiling alcohols, 2 nonane, ethylene glycol monethylether and its ester derivatives, etc., methanol being preferred. Thegelled membrane, still adhered to the silane-modified mold member isthen baked and stripped from the mold. Empin'cally, it has been foundthat the membrane possesses the requisite pore size distribution andconforms substantially identically to the deformations originallyimparted to the heretofore removed base material.

As mentioned above, the polymeric materials used in forming themembranes of the present invention are high molecular weight,film-forming polymers of vinylidene fluoride, the homopolymers beingpreferred. As illustrative of these materials, mention is made of thepolyvinylidene fluoride polymeric materials commercially available fromPennsylvania Salt Manufacturing Company, 3 Penn Center Plaza,Philadelphia, Pennsylvania, under the trade name Kynar" (a homopolymercontaining 59 percent fluorine). Such material may be obtained and used,for example, in the form of a 5 micron size particle powder under thename Kynar" 301 powder.

The copolymeric materials which may be used in the present inventioncontain a major proportion of vinylidene fluoride and, preferably willcontain at least about 90 percent vinylidene fluoride. The materialswhich may be copolymerized with the vinylidene fluoride areethylenically unsaturated materials which preferably have no functionalgroup other than the group. Such materials may be illustrated byethylene, propylene, butylene, vinyl chloride, vinyl fluoride,vinylbromide, vinylidene chloride, ethyl acrylate, methyl methacrylate,etc. The term polymeric vinylidene fluoride as used in the appendedclaims is intended to include the aforementioned copolymeric materialsas well as the preferred homopolymeric material as mentionedhereinbefore.

To form the films of the present invention it is necessary to form asolution" of the polymeric vinylidene fluoride, as aforementioned.Polymeric vinylidene fluoride does not form true solutions with asolvent but, as indicated in US. Pat. No. 3,2l 1,687 to Caperon et al.forms pseudosolutions with a number of solvents. Such solvents comprisedimethyl acetamide, dimethyl sulfoxide, tetrarnethyl urea, diethylacetamide, and mixtures thereof, the preferred material being dimethylacetamide. These solvents can be used to form suitable pseudosolutionswhich at room temperature can be effectively formed into polymericvinylidene fluoride films. While other materials can be used to effect asolvent solution of the polymeric vinylidene fluoride, at, for example,high temperatures, many such solvents produce solutions which gel atambient temperatures and accordingly are of lesser practical value inpreparing the film of the present invention.

The solution of polyvinylidene fluoride, to be used in the presentinvention, may be prepared by placing a selected solvent in a containerequipped with an agitator, heating the solvent at a temperature withinthe range of about 28 to 62 C. and adding the polyvinylidene fluoride,in powder form, to the heated agitated solvent. Alternatively, thepolymeric vinylidene fluoride may be admixed with the solvent prior toheating to 28 C. to 62 C. The heating and agitation is continued, ineither case, until apparent solution is obtained. Many types ofapparatus for effecting the solutions, under the conditions specified,are well known and these may be illustrated by a paint mill or a colloidmill. Raising the temperature of the polyvinylidene fluoride solution,prior to casting into a film, to 34 C., has been found to produce amembrane possessing a pore size distribution, predominantly in the 0.5to 1.0 micron range. It will be accordingly appreciated thatmicropermeable membranes comprising polyvinylidene fluoride withsubstantially any desired pore size distribution may be formed byselecting an appropriate maximum temperature to which the polyvinylidenefluoride solution is elevated prior to casting, which is more fullydiscussed in the Bailey and McCune application, supra.

As reported in the last-mentioned Caperon et al., patent, the solventscan dissolve and form solutions containing as high as 30 percent, byweight, polymeric vinylidene fluoride. However, for forming themicroporous films of the present invention, it is preferred that thesolutions have a percent polymer in the range from about 15 to 25percent, by weight, and most preferably, 20 percent by weight.

The polyvinylidene fluoride solution is applied to the silane modifiedsurface of the mold member by any suitable coating technique. It ispreferred to doctor the material onto the mold member to provide asubstantially consistent thickness throughout the membrane when measuredat either the peak of triangle of the convolutions, respectively.Optimum thickness on a wet basis at the convolution peaks, is about0.020 inch, which produces a membrane approximately 4.5-6.5 mils. inthickness on a dry basis, again measured at the convolution peaks. Asdenoted above, the mold member with the adherent polyvinylidene fluoridesolution is contacted with in a liquid which is a nonsolvent for thepolyvinylidene fluoride, but which is miscible with the originalpolyvinylidene solvent. The composite mold member-membrane structure isallowed to remain in contact with the nonsolvent until the gel structureachieves sufficient physical strength and substantial extraction of thepolyvinylidene fluoride solvent is assured. Approximately 10 minutes maybe employed for this operation.

Next, the mold member with the adherent membrane is oven-dried and theultimate corrugated membrane is stripped from the mold member. it hasbeen found that stripping may be facilitated if it is carried out in anaqueous environment.

The 4.5 to 6.5 -mil thick membrane is considered to be of sufficientthickness to be self-supporting but is not so thick that vaportransmission efficiency characteristics of the membrane would bedetrimentally affected. ideally, the oven-drying step will be executedat a temperature insufficient to cause the polymeric vinylidene fluorideto fuse its fusion temperature being approximately l75 C. The bakingoperation anneals the film thereby removing casting strains and sets thefilm in a fixed geometrical structure. A baking time of about fifteenminutes generally is sufficient to provide the desired results.

The instant invention will be better understood by a consideration ofthe example which follows. Included in the example will be a discussionof the figures of the drawings which sets forth the general proceduresto be followed in the practice of the present invention; illustratesmembranes made according to the present invention both with and withoutsilane modification of the mold member; and demonstrates the pore sizedistribution of membranes of the present invention.

EXAMPLE 1 A polyvinylidene fluoride membrane having a pore volume ofapproximately 70 percent wherein at least 75 percent of the pores have apore diameter from between 0.5 to 1 micron is prepared according to thefollowing procedure:

A l-mil thick sheet of Lexan" polycarbonate material is run through anembossing apparatus comprising a heated embossing die possessing 72convolutions of about 5 mil. depth per inch and a silicone rubberbacking roll. The embossed Lexan is then swab-coated with a 2 percentsolution of yaminopropyltriethoxysilane designated as "Al inisopropanol. The silane coated Lexan is allowed to air dry for 30minutes. Next, a glass'plate IS doctor-coated with a room temperaturevulcanizable silicone rubber designated as GE Type I02" in a thicknessof approximately one-thirty second of an inch lmmediately, the silanemodified surface of the Lexan base material is contacted with the roomtemperature vulcanizable silicone rubber and the rubber is allowed tocure at room temperature by placing it in an environment kept atapproximately 80 C. for 2 4 hours. The entire composite structure isthen immersed in a bath of methylene dichloride which dissolves theLexan from the structure thereby producing a silane modified siliconerubber mold.

A 20 percent by weight solution of polyvinylidene fluoride is preparedby mixing g. of "Kynar 301" powder in 40 g. of dimethyl acetamide at 34C. The mixture is stirred for 1 hour to assure solvation of thepolyvinylidene fluoride. The polyvinylidene fluoride is then doctoredonto the silane modified mold member in a wet thickness of 0.020 inch,and immersed in a nonsolvent bath of methanol for 10 minutes. Afterremoving it from the methanol the membrane-mold structure is air-driedfor 30 minutes and baked in an oven at l50 C. for minutes. The membraneis then physically stripped from the mold member and possessedconvolutions substantially identical to those imparted to the originalLexan base material.

FIG. 2 is a photomicrograph of the membrane prepared according to thepresent example and evidences well-defined convolutions which arebeneficial for use in desalination apparatuses as denoted above. On theother hand, a repetition of the instant example, without the silanemodification step, produces the membrane depicted in FIG. 3. The absenceof definition of the convolutions is considered to be due to a prematureseparation of the gel structure from the mold member.

The pore size distribution of the product of example 1 has beendetermined using the Skau-Ruska high pressure mercury intrusion test,employing an Aminco-Winslow Porosimeter commercially available fromAmerican Instrument Co., Silver Springs, Md., and the data from the testis graphically reproduced in the curve of FIG. 4. The curve representsthe plotting of the intrusion of mercury in ccs, as the ordinate,against the pressure applied to the mercury in p.s.i., as the abscissa.The abscissa may be mathematically converted to represent pore sizediameter employing the expression d =2r where d is pore size diameter inmicrons (pi) and r =2 'ycos. O/P wherein y is the surface tension ofmercury (Hg), i.e., 69.6181 pounds microns per square inch; 0 is thecontact angle of mercury against the microporous polyvinylidenefluoride, i.e.,

about 130; and p is the pressure applied in pounds per square inch- Thecurve clearly indicates, by the area beneath the peak, that over 75percent of the pores within the film have a pore size within the rangeof 0.5 to 1 micron.

While the present invention has been directed toward a microporouspolymeric vinylidene fluoride film particularly adapted for use in thedistillation apparatus of the aforementioned copending application Ser.No. 456,040,the invention is in no way limited thereto. Such microporousfilms may also be used in other areas where microporous films areconventionally employed, such as microbiological applications; thefiltering of oil used in the lubrication of miniature bearings; thefiltering of rocket engine fuel; organic solvent filtration, etc. Thepresent invention specifically provides a microporous film ofpolyvinylidene fluoride which exhibits a sufficiently high degree ofpore size uniformity and pore volume so as to provide for the efficientemployment of the film in various filtration or liquid purificationprocesses.

Since certain changes may be made in the above process and productwithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description and shown inthe accompanying drawing shall be interpreted as illustrative and not ina limiting sense.

What is claimed is:

l. A corrugated micropermeable membrane containing at least eightconvolutions per inch comprising a pol mer contaming at least percentvinylidene fluoride an having a pore volume of at least 50 percentwherein a majority of said pores have a pore diameter of from about 0.5to 2 microns.

2. The invention of claim I wherein at least 75 percent of said poreshave a pore diameter of from approximately 0.5 to 1.0 microns.

3. The invention of claim 1 wherein less than 5 percent of the pores ofthe said membrane have a diameter greater than 2.0 microns.

4. The invention of claims 3 wherein the thickness of said membrane isbetween about 4.5 to 6.5 mils when measured at the convolution peaks.

5. The invention of claim 4 wherein the amplitude of said convolutionsis approximately one-half the thickness of the membrane.

6. The invention of claim 5 wherein the corrugation comprisesapproximately 72 convolutions per inch.

7. The invention of claim 6 wherein the thickness of said corrugatedmembrane is 6 mils when measured at the convolution peaks.

1' i i t i

2. The invention of claim 1 wherein at least 75 percent of said poreshave a pore diameter of from approximately 0.5 to 1.0 microns.
 3. Theinvention of claim 2 wherein less than 5 percent of the pores of thesaid membrane have a diameter greater than 2.0 microns.
 4. The inventionof claims 3 wherein the thickness of said membrane is between about 4.5to 6.5 mils when measured at the convolution peaks.
 5. The invention ofclaim 4 wherein the amplitude of said convolutions is approximatelyone-half the thickness of the membrane.
 6. The invention of claim 5wherein the corrugation comprises approximately 72 convolutions perinch.
 7. The invention of claim 6 wherein the thiCkness of saidcorrugated membrane is 6 mils when measured at the convolution peaks.